Slitting machines have long been used to slit webs of material into strips of various widths. Such machines are used to slit webs of metal to form strips that will become everything from razor blades to electric conductors for integrated circuit computer chips. Slitting machines have a pair of arbors on which are mounted opposing pairs of knives. The knives of each pair of cutters are positioned along the length of the arbors so that they shear the metal as it passes through the machine. Positioning of the knives is accomplished by placing each knife on the arbor and separating it from its neighbors with spacers of controlled width. The stack of knives and spacers on each arbor is held in place by a shoulder at one end of the arbor and a nut which compresses the stack against the shoulder from the other end of the arbor.
The position of the knives on each arbor is critical, since too wide a space between the opposing knives of each pair means that the metal strips will have burrs on the edges, while too narrow a space means that the cutters will interfere with each other or wear too rapidly. Also, having too wide of a space on one side and too narrow a space on the other side of a slit strip induces camber into it. The spacing for knives for thin materials is difficult to maintain accurately where a wide web of thin material is to be slit into narrow strips. For example, in slitting webs of metal 0.005 inches thick, such as might be used for contact strips in an integrated circuit computer chip, each knife must be positioned to within .+-.0.0005 of an inch. Since the web being slit may be two feet wide or more and the strips being cut may be one half inch wide or less, there are a large number of knives and spacers. In order to maintain the position of each knife with the desired accuracy (.+-.0.0005 inches) it is necessary to machine the spacers and knives to an accuracy of .+-.0.000040 inches (forty millionths of an inch). If such precision is not maintained, then the errors in positioning each knife cumulate across the width of the arbor to a degree that exceeds the necessary accuracy to produce a good finished product.
The need for accurate positioning has been recognized in the past, and various techniques have been used to achieve it. Precision machining operations have been used and special care in handling has been used in the manufacture and installation of the knives and spacers. This is costly and time consuming. However, even the most careful installation of precisely machined knives and spacers will not achieve the desired results if the arbors to which the knives and spacers are mounted are not accurately positioned with respect to each other. Thus it becomes a matter of importance for the arbors to be accurately positioned with respect to the machine's frame and so with respect to each other.
Any attempt to position the arbors accurately relative to each other begins with quality bearings to support the arbors and the various moving parts which surround them in the machine's frame. Included in such parts are eccentrics which support the bearings in which the arbors turn. These eccentrics are necessary to allow the arbors to be moved away from each other so that the knives can be installed on each arbor without interfering with the knives on the parallel arbor. Thus there are two sets of bearings which wear, those between the eccentrics and the machine frame, and those between the arbors and the eccentrics. As these wear, play increases, and the arbors' axial positions become less and less fixed. Precision bearings are not sufficient to eliminate wandering of the arbors because they wear and inevitably have some play which makes accurate positioning of the arbors difficult. One solution to this problem is illustrated in U.S. Pat. No. 4,667,550. There the axial wandering of the arbors is controlled by a feedback system that senses the relative position of the arbors and moves one arbor relative to the other arbor to correct for deviation from the desired location.